Hi folks! Life long sailor - soon to be kayaker here.
Having decent sailshape going downwind is always a challenge. The forces of apparent wind against the front of the sail battling with the wind behind the sail work against each other. So, yeah, a whisker pole is a great idea in my opinion. There is more to just "holding the sail out" though. Of course
The purpose of the pole is to try and maintain a good "airfoil" shape as well as to keep the sail on a preferred side when running dead down wind. Note of Caution: in a heavy blow DO NOT sail DEAD DOWN WIND, always sail a little off the wind to keep the sail over on one side. It is a real pain in the head if a gust lifts the sail suddenly across the boat because it "wants" to be on the other side.
A whisker pole should be light and fairly stiff. You don't want to add any more weight to the clew of the sail than you need to. A full sail in light conditions should be able to "lift" the pole when you let go the sheets. For example: on a typical 25 foot 5000lb sailboat we only use a 1 ¼ inch diameter, 6 to 12 foot twist lock pole. So work that backwards into all of what you need for a 50 lb kayak. For optimum effect the pole should be extendable. I’m thinking a small boat telescoping tiller extension or a leg from a small camera tripod or the shaft of your kayak paddle could be adapted.
Here is a site for some whisker poles and parts info. You'll probably find a lot of other "boating" stuff too that will have your credit card maxed up in no time
http://www.westmarine.com/webapp/wcs/st ... /342/341/2